1. Field of the Invention
The present invention relates to an illumination device that employs light emitting diodes (LEDs), an image reading device that reads images of an original document by illuminating the original document with the illumination device, and an image forming apparatus incorporating the image reading device and equipped with copying capabilities such as monochromatic copying, full color copying, multi function capabilities, etc.
2. Description of the Related Art
FIG. 9 is a schematic diagram of a conventional image reading device (hereinafter, “scanner”) that scans images of an original document. A scanner 200 basically includes a document plate (contact glass plate) 11 on which the original document is placed, a first carriage 3 disposed below the document plate 11 and including a light source 1 and a first mirror 2, a second carriage 6 consisting of a second mirror 4 and a third mirror 5, an imaging lens 7 on which a reflected light from the original document that has been guided through the first mirror 2, the second mirror 4, and the third mirror 5, and a charge-coupled device (CCD, which is an imaging element) 8 that reads and carries out photoelectric conversion of the image formed on the imaging surface by the imaging lens 7. In the conventional scanner 200, when scanning the original document, the second carriage 6 moves at half the scanning speed of the first carriage 3 in the sub-scanning direction.
FIG. 10 is a perspective of an illumination device that uses a xenon lamp as a light source. As shown in FIG. 10, the light source 1 of the first carriage 3 is a cylindrical xenon lamp 9, which is disposed within a housing 13 having an opening 12 towards the contact glass plate 11. The light emitted directly from the xenon lamp 9 is directed towards the contact glass plate 11. Additionally, a reflective plate 10 provided near the exit of the housing 13 also deflects the light emitted from the xenon lamp 9 towards the contact glass plate 11. The first carriage 3 guides the reflected light from the high intensity area to the second mirror 4 and the third mirror 5. When this kind of light source is used, the imaging position is different for each of the colors R, G, and B, as shown in FIG. 11, requiring uniform distribution of light intensity on the surface of the original document enough to cover the difference.
Light emitting diode (LED) is being looked into to serve as a light source (point light source) from the viewpoints of energy saving, quick starting, and reliability. However, the LED, when used alone, has demerits like degradation of light intensity, rippling (unevenness of light intensity) in the main scanning direction, etc., and therefore cannot be used as a light source as it is.
Therefore, the LED is coupled with a light guide. Japanese Patent Laid-Open Publication No. 2000-48616 discloses an illumination device, which is provided with an iris disposed between the light source and the light guide that causes the light beam from the light source to enter the light guide at an angle that is equal to or greater than a critical angle and be directed towards a diffusing unit. In the invention disclosed in Japanese Patent Laid-Open Publication No. 2000-48616, the light guide is a rod-like structure composed of a transparent material whose end surface functions as the entrance surface and at least a portion of whose side surface functions as the exit surface. The iris restricts the light beam entering from the entrance surface such that the beam forms an incidence angle, which is equal to or greater than a critical angle the beam forms with the side surface.
However, in the illumination device disclosed in Japanese Patent Laid-Open Publication No. 2000-48616, the relation between the characteristics of LED and the light guide is not defined.
In other words, as a narrow-directivity LED is used as the light source for scanning an image the LED, a peak occurs in the light distribution pattern as shown in FIG. 12, and the light intensity distribution on the surface of the document across the scanning width is neither uniform nor sufficient. Narrow-directivity LED also gives rise to significant unevenness of light intensity in the main scanning direction, leading to what is known as rippling. An ideal LED would be a wide-directivity LED having an illuminating beam that spreads out in the main scanning direction and casts uniform light intensity across the scanning width in the sub-scanning direction, as shown in FIG. 13.
However, in the wide-directivity LED, when the light guide guides the light emitted from the LED towards the surface of the document, the light rays that come out without being totally reflected escape outside from the light guide, leading to loss of the light and attenuation of the light intensity.